if your shackle is laying back to far it will over stress your springs and cause premature failure. The hole point of the shackle is to allow the spring to stretch out. If you aren't allowing it to stretch out properly if will cause the spring to find other ways to stretch (ie. S'ing of the spring). Thats why on long travel springs you sometimes need to put longer shackles on, because the spring has more arch and therefor requires a greater distance to flex out.
Hope that makes sense I'll explain it better if it didn't.
